1. Field of the Invention
The present invention relates to an exhaust gas purification method and an exhaust gas purification system in which an unburned fuel supplied by post injection or the like into an exhaust gas in an exhaust passage of an internal combustion engine is oxidized by an oxidation catalyst and using the resulting oxidation heat, a temperature of a diesel particulate filter is raised for regeneration.
2. Description of the Related Art
Regulations on the amount of particulate matters (PM: particulate matter: Hereinafter referred to as PM) exhausted from a diesel engine as well as NOx, CO, HC, and the like have been tightened year by year. A technology has been developed that the PM is collected by a filter called a diesel particulate filter (DPF: Diesel Particulate Filter: Hereinafter referred to as DPF) to thereby reduce the PM amount exhausted to the outside. A continuous regeneration type DPF device carrying a catalyst is among them.
In this continuous regeneration type DPF device, when an exhaust gas temperature is approximately 350° C. or above, the PM collected by the filter is continuously burned and purified, and the filter is self-regenerated. However, if the exhaust temperature is low, the temperature of a catalyst is lowered and the catalyst is not activated. Thus, the oxidation reaction is not promoted, and oxidation of the PM and regeneration of the filter become difficult. As a result, accumulation of PM on the filter makes clogging of the filter progress, which results in a problem of exhaust pressure rise due to the clogging of the filter.
One of methods to solve such a problem is forced regeneration control for forcedly burning and removing the collected PM by forcedly raising the temperature of exhaust gas when the clogging of the filter exceeds a predetermined amount. In this forced regeneration control, the exhaust gas temperature rise control is conducted so as to raise the temperature of the exhausts gas flowing into the filter to a temperature or above at which the PM collected by the filter is burned.
This kind of exhaust gas temperature rise control includes methods of conducting multiple injection (multiple-stage delayed injection) or post injection in the cylinder (in-cylinder) fuel injection and a direct injection method into an exhaust pipe or the like. The multiple injection is a delayed multiple-stage injection in which the fuel is injected into the cylinder in many stages. By this multiple injection, a fuel amount simply burned in the cylinder without generating torque is increased, and the temperature of the exhaust gas exhausted from the cylinder is raised to a catalyst activation temperature or above of the oxidation catalyst. The post injection is injection for auxiliary injection at timing further delayed from the multiple injection after conducting main injection in the in-cylinder injection. By this post injection, HC (hydrocarbon), which is an unburned fuel, is increased in the exhaust gas exhausted from the cylinder and the HC is oxidized by the oxidation catalyst, by which the temperature of the exhaust gas on the downstream of the oxidation catalyst is raised.
At this exhaust temperature rise, if the exhaust gas temperature is low such as in a case of a low-load/low-speed operation state, the temperature of the exhaust gas flowing into the oxidation catalyst is raised to a catalyst activation temperature or above of an oxidation catalyst by conducting the multiple injection in the first place. Then, after the temperature of the oxidation catalyst has been raised to the catalyst activation temperature or above, the post injection is conducted while the exhaust gas temperature is maintained at the catalyst activation temperature or above, and HC is supplied to the oxidation catalyst. This HC is oxidized by the oxidation catalyst and generates heat, and the exhaust gas flows into the filter in a state where the temperature is further raised. By this high-temperature exhaust gas, the PM accumulated in the filter is burned and removed.
However, if the post injection amount is larger than an amount that can be oxidized by the oxidation catalyst, the unburned fuel is not oxidized but flows out to the downstream side of the exhaust gas purification system, and white smoke is generated. Thus, in order to prevent generation of such white smoke, the fuel injection amount in the post injection is finely controlled so that the post injection amount does not exceed the amount that can be oxidized by the oxidation catalyst.
Examples of the exhaust gas purification system proposed for conducting this control include an exhaust gas purification device in which an oxidation catalyst is arranged on an upstream side and a catalyst regeneration type particulate filter on a downstream side as described in Japanese Patent Application Kokai Publication No. 2004-143988, for example. In this device, a temperature sensor for detecting an exhaust temperature is disposed between the oxidation catalyst and the catalyst regeneration type particulate filter. According to a detected temperature of the temperature sensor, gradual change control is conducted so that the injection amount at the start is increased in a stepped manner and the exhaust gas temperature is raised early to a target temperature. Then, proportional-integral control is conducted so that the exhaust gas temperature is stably maintained at the target temperature for a predetermined time and accurate and fine temperature control is conducted.
In these exhaust gas purification systems, a supply amount of an unburned fuel which can be oxidized by the oxidation catalyst is calculated from the exhaust gas temperature relating to the temperature of the oxidation catalyst. However, since a combustible fuel amount which can be oxidized by the oxidation catalyst also relates to an air excess ratio (or air/fuel ratio) relating to an oxygen amount and an atmospheric pressure, it is not sufficient to simply calculate an upper limit value of the unburned fuel supply amount by a catalyst temperature index temperature indicating the oxidation catalyst temperature. Therefore, generation of white smoke can not be fully prevented.
Patent Document 1: Japanese Patent Application Kokai Publication No. 2004-143988